Digital radio for carrier telephony appeared in the early 1970's and was limited to modest spectral efficiencies and relatively short distances. The field has greatly grown over the past decades and the use of digital radio is widespread. Spread spectrum systems and, in particular, frequency hopping and direct sequence transmission systems, have produced results in communications, navigation and test systems that were not possible with standard signal formats. Frequency hopping signal transmission systems are a type of spread spectrum system in which the wideband signal is generated by hopping from one frequency to another over a large number of frequency choices. The frequencies used are chosen by a code similar to those used in direct sequence systems. In direct sequence systems a radio frequency (RF) carrier is modulated with two data streams in quadrature to produce a signal that is 0 degrees (in phase) when the data stream code represents a particular binary number (e.g. a "zero") and a signal shifted in phase when the data stream code represents the other binary number (e.g. a "one"). The SINGCARS (Single Channel Ground Airborne Radio System) radio system is a type of frequency hopping system that hops through the 30-88 MHZ band for transceiving voice and data communications in a variety of modes. For general background on spread spectrum systems, reference is made to a text entitled Spread Spectrum Systems, 2nd edition, by Robert C. Dixon and published by Wiley-Interscience, New York (1984). In order to increase the efficiency of digital radios employing spread spectrum characteristics, digital engineers have raised the number of modulation levels and have generally dealt with modulation/demodulation techniques, spectral shaping and synchronization schemes. This has led to widespread and more efficient use of the digital radio systems.
During the past decade, many improvements have been implemented involving advanced digital radio techniques. Digital radio is used both commercially and for the military. However, with the increased use and applications for digital radio systems, new designs for more compact, lightweight radios providing enhanced processing modes, greater fidelity and lower power are greatly desired. Past designs for two way radios have used multiple low performance microcontrollers for implementing the functionality of the radios. However, these constructions do not permit the use of high level language software for implementing and executing the various radio functions. Other designs employ microcontrollers in combination with customized hardware. However, this solution suffers by being relatively inflexible to adding new features and modes once a hardware configuration is finalized. Furthermore, many hardware implementations of radio functionality require a large amount of real estate to provide the necessary circuit configurations. In addition, noise resulting from stray signal interference and coupling between baseband and RF signals pose a further problem in current radio designs. Consequently, a low power software based radio architecture providing greater ease, flexibility, and reconfigurability for enhancing current operating modes and sophisticated features of digital radios while providing isolation between baseband and RF signals for minimizing interference is greatly desired.